1. Field of the Invention
The present invention relates generally to radiant-energy fuse structures which are configured to customize electronic devices, and more particularly, to a radiant-energy configurable fuse structure having a non-uniform width.
2. Description of Related Art
Configurable fuses are widely used in the electronics industry for the configuration or repair of elements in memory and logic circuits. These fuses are made from conductive materials such as polysilicon, silicides, and metals. The fuses are typically formed on and covered by one or more insulating layers such as silicon oxide or silicon nitride, thus encapsulating the fuse. During configuration, selected fuses are typically irradiated from above by a radiant energy beam, which is generally circular or elliptical in shape and provides a measurable radiant energy. The most common means of providing this energy is through a laser beam, such as the Model 1225hp manufactured by Electro-Scientific Incorporated (ESI) of Portland, Oreg. The radiant energy heats the material in the fuse until a portion of it becomes a liquid, a vapor, or a combination thereof (hereinafter referred to as a liquid/vapor state). Simultaneously, heat propagates along the length of the fuse from the irradiated to the non-irradiated sections. The change in the fuse material state from a solid to a liquid/vapor state causes pressure to build up in the cavity that contains the fuse. This results in a rupturing of the insulator, typically along the structurally-weakest point. At least a portion of the fuse material escapes through this ruptured area. The thickness of the insulator over the fuse is usually chosen to be thinner than the film surrounding the fuse so that the insulator portion over the fuse will be the first area to undergo structural failure when the pressure reaches a critical value. In this manner, the insulator over the fuse will be blown off and the force of the explosion directed away from neighboring critical circuit elements. It is common for the thickness of the insulator to be set to a value that provides the most consistent results in disconnection of the fuse (hereafter generally referred to as fuse blowing).
FIG. 1 illustrates a top plan view of a conventional electronic device having conventional configurable fuses 1. Each of fuses 1 has a fuse body 2, which is typically the same width or narrower than connection terminals 3 connecting conductive lines 4 and 5 to underlying elements. A radiant energy beam 7 severs the connection between line 4 and line 5 by directing the beam at fuse body 2. Because the thermal conductivity of the fuse material greatly exceeds that of a typical insulator which encloses the fuse, the majority of the transmitted heat from the radiant energy beam 7 is conducted through the connection terminals 3, and not the insulator. Thus, the connection terminals 3 must be long enough to protect lines 4 and 5 from damage as a result of thermal conduction, which adds to the spacing between lines 4 and 5. While the additional length protects the lines, a significant amount of transmitted heat is also drawn away from the fuse body. The overall energy required to blow the fuse must therefore be increased to compensate for this loss. However, since beam 7 typically has a radial energy distribution which is approximately Gaussian in nature, increasing the beam energy necessarily increases the effective spot size 6 of beam 7 (effective spot size being defined as the diameter of the area of radiant energy beam which could affect active circuit elements). Thus, as the energy increases, adjacent ones of fuses 1 must be spaced further apart. Consequently, the packing density of the conventional fuses 1 is relatively low, requiring significant layout area when compared with simple non-configurable interconnect lines. The result is an undesirable lower density and larger size device.
With a conventional fuse design as shown in FIG. 1, some of the energy is captured along the length of the fuse body 2, while much of the remainder is lost outside the much narrower fuse body width. The energy not captured by the fuse body 2 is then free to travel past the fuse to any underlying circuit elements which might be routed below it. Therefore, it has been common for the area around and under the fuses to be clear of circuit elements (e.g., transistors, resistors, signal lines, junctions, etc.) so that the radiant energy used to configure the fuses does not damage these circuit elements. The addition of this clear area increases the overall size of the device, as does the additional area required to place or route circuit elements away from the fuse area.
The conventional fuse design is also subject to inconsistency in fuse blowing. Variations in the thickness of the fuse material and the insulator over the fuse can result in the fuse blowing earlier or later in the fuse blowing process and producing irregularly sized and shaped holes from which the fuse material escapes. Thus, the fuse material may not be sufficiently vaporized to complete the disconnection, or the surrounding insulator may rupture, thereby damaging neighboring circuit elements.
The above manufacturing constraints seriously complicate how an electronic device can be laid out and effectively increases the area for the circuit elements of the device.
Accordingly, it is desirable to provide a radiant-energy configurable fuse that overcomes the deficiencies discussed above with respect to conventional fuses.